Field of the Invention
Embodiments described herein relate generally to a pattern inspection apparatus. Embodiments described herein relate, for example, to a pattern inspection technique for inspecting pattern defects in an object to be a target object used to manufacture a semiconductor, and to an inspection apparatus that inspects an exposure mask substrate used in manufacturing a semiconductor element and a liquid crystal display (LCD).
Related Art
In recent years, the circuit line width required for semiconductor elements is getting even smaller due to increasing integration level and increasing capacity of a large-scale integrated circuit (LSI). These semiconductor elements may be manufactured by forming a circuit by transforming a pattern on a wafer through exposure by a reduction projection exposure apparatus, which is a so-called stepper, using an original pattern having a circuit pattern formed thereon (also called a mask or a reticle, and collectively called a mask hereinafter). Therefore, in order to manufacture a mask for transferring such a fine circuit pattern to a wafer, a pattern forming apparatus using electron beam, which can forma fine circuit pattern, is used. By using this type of pattern forming apparatus, a pattern circuit may be directly formed on a wafer. Other than electron beam, development of a laser beam forming apparatus, which forms a pattern using laser beam, has been attempted.
For manufacturing LSI requiring high manufacturing cost, improvement of yield is essential. However, as represented by a gigabit class dynamic random access memory (DRAM), a pattern for an LSI is in the order of submicron to nanometer. One of main factors for degradation of yield may be pattern defects in a mask used in transferring an ultrafine pattern to a semiconductor wafer by exposure of a photolithography technique. In recent years, a size of pattern defects that have to be detected becomes very small as size of an LSI pattern formed in a semiconductor wafer becomes finer. Therefore, high precision of a pattern inspection apparatus that inspects defects in a transfer mask used to manufacture LSIs is demanded.
As an inspection technique, there is a known method of inspecting by comparing an optical image obtained by capturing an image of a pattern formed on a target object such as a lithography mask with a predetermined magnification using an expansion optical system with an optical image obtained by capturing an image of the same pattern in design data or on a target object. Examples of pattern inspection methods include “die to die inspection” and “die to database inspection”. In “die to die inspection”, pieces of optical image data obtained by capturing an image of the same pattern at different positions on the same mask are compared. In “die to database inspection”, pattern forming data (design pattern data) is obtained by converting format of CAD data of a designed pattern to an apparatus input format to be input by a forming apparatus when the pattern is formed using the CAD data as a mask, the pattern forming data is input to an inspection apparatus, the inspection apparatus generates a design image (reference image) based on the pattern forming data, and the design image is compared with an optical image serving as measurement data obtained by capturing an image of the pattern. In the inspection method performed by such an inspection apparatus, a target object is placed on a stage, and the target object is scanned with a luminous flux to inspect the target object. The target object is irradiated with a luminous flux by a light source and an illumination optical system. Light that has been transmitted through the target object or reflected by the target object forms an image on a sensor through the optical system. The image captured by the sensor is transmitted to a comparator circuit as measurement data. The comparator circuit compares the measurement data with the reference data with an appropriate algorithm after alignment of the images, and determines that a pattern defect is present when the images do not match.
Generally, in order to acquire a high-resolution image, an imaging system of an inspection apparatus has become a magnifying system. Thus, lenses having long focal distance have to be combined, necessarily resulting in a long optical path. Therefore, the imaging system is susceptible to the influence of air fluctuation. Since air fluctuation corresponds to variation of refractive index distribution in an optical path, air fluctuation works as an effect of bending a ray of light. This results in displacement of a pattern image on an imaging sensor. This displacement causes an error when the pattern image is compared with a reference image, and thus is a factor for preventing highly accurate inspection. Therefore, it is desirable that such displacement be measured. In regards of this issue, it is suggested to provide a four-divided sensor or the like as a sensor for measuring displacement separately from a sensor for imaging a mask pattern, image a cross pattern for measurement by the four-divided sensor or the like, and calculate the displacement (refer to, for example, JP 2011-257164 A).